Calculate radiant heat output and surface temperatures. Understand the physics of heat transfer to ensure comfort and safety in your living space. Modeling radiant energy helps in determining the most effective placement for your stove and nearby furniture.
Enter the stove's surface temperature, its radiating area, the surface material, and the room temperature. The calculator evaluates the Stefan–Boltzmann law, Q = ε·σ·A·(Ts⁴ − Ta⁴), with σ = 5.67×10⁻⁸ W/m²K⁴ and both temperatures converted to kelvin. Output rises with the fourth power of absolute temperature: lifting the same 1.5 m² cast-iron surface from 150°C to 250°C nearly triples its net radiation, from about 1,994 W to about 5,455 W.
Emissivity sets how close a surface comes to an ideal radiator: cast iron 0.95, soapstone 0.90, and bare steel 0.70. Material alone is worth hundreds of watts — at a 250°C surface in a 20°C room, the 1.5 m² example emits roughly 5,455 W in cast iron but only about 4,020 W in steel, a 26% difference with identical geometry and temperatures.
Intensity at one metre divides the output over a hemisphere (Q ÷ 2π, about 868 W/m² in the example) since stoves radiate mainly forward. The comfort distance then solves for the radius where the radiant flux falls to 10 × (43°C − room temperature) W/m², treating 43°C as the threshold for prolonged comfortable exposure — about 1.94 m for the example stove in a 20°C room.
Each material has an emissivity between 0 and 1 describing how efficiently it radiates compared with a perfect black body. Cast iron at 0.95 is close to ideal, soapstone follows at 0.90, while bare steel at 0.70 reflects more and radiates less. In the Stefan–Boltzmann equation emissivity multiplies everything else, so switching the same hot surface from cast iron to steel trims radiant output by about a quarter.
It is the radius at which the modeled radiant flux drops to a threshold of 10 W/m² for every degree between 43°C and your room temperature. In a 20°C room that threshold is 230 W/m². Because the threshold shrinks as the room warms, the same stove projects a larger comfort radius in an already-warm space: at 30°C the allowable flux halves to 130 W/m² and the computed distance grows by about a third.
Radiation scales with absolute temperature to the fourth power, so gains compound quickly. Moving the example surface from 150°C (423 K) to 250°C (523 K) raises the Ts⁴ term by a factor of about 2.3 and, after subtracting the room's back-radiation, lifts net output from roughly 1,994 W to 5,455 W. This is also why an over-fired, glowing stove dumps so much more heat onto nearby surfaces than one in its normal operating band.